Arrays of electronic computers, such as are found in data centers, generate a great deal of heat. An example Central Processing Unit of a computer (“CPU”) generates over 100 watts of heat and has a maximum case temperature of about 60 C. An example rack of 88 CPUs may generate 9 kW of heat. The outdoor temperature at a hot urban location might be 45 C, so even in hot environments heat can still theoretically flow away from the higher temperature computer and toward the lower temperature outside environment. Accordingly, no refrigeration of computers should be required, theoretically. Nonetheless, the standard way to keep data centers cool is to use expensive and relatively inefficient vapor-compression refrigeration systems at least part of the time. These conventional cooling or “air conditioning” systems often use more power that the computers themselves, all of which is discharged to the environment as waste heat. These systems use air as the heat transfer medium, and it is due to the low heat capacity and low thermal conductivity of air that refrigeration must be used to remove the heat generated by multiple air heat exchangers. Removing heat generated by heat exchangers is also referred to as overcoming the thermal resistance of the heat exchangers. Some operators use evaporation of cooling liquid to cool cooling liquid-to-air heat exchangers that cool computers, and this is more thermally efficient than refrigeration, but the computers run hotter, reducing their reliability, decreasing their efficiency and making the data center uncomfortable for human occupants. Water is used as the cooling liquid or coolant throughout this disclosure, but it will be known to those in art that other coolants may be used. The cooling liquid may consist essentially of water, including tap water, or may comprise one or more perfluorocarbons or avionics cooling liquids. The cooling liquid may flow over a plated surface.
Water has approximately 4000 times more heat capacity than air of the same volume, so water is a theoretically ideal heat transfer agent for direct heat transfer from heat generating components. Other cooling liquids offer similar performance. Liquid cooling is recognized as a thermally efficient way to cool computer CPUs due to their high concentration of power and heat generation in a small space, but the rest of a computer's electronics generate heat at a lower rate and temperature, so air-cooling is appropriate for much of the associated hardware. Current systems may use liquid cooling to move the heat from the CPU to a radiator mounted close to the CPU, or they may use an air-to-liquid heat exchanger to remove heat from the computer enclosure and heat-up liquid in the heat exchangers. These systems suffer from the high thermal resistance and bulkiness of air-to-liquid or liquid-to-air heat exchangers. Other systems use a chilled cooling liquid loop to cool the computer, but these systems require complex and expensive connectors and plumbing to connect the server to the building cooling liquid supply while insuring that no leaks occur, which may be devastating in or near a computer. Accordingly, operators of server systems are rightly concerned about leaks and reliability of cooling liquid-cooled computers. Furthermore, chillers require a large amount of power. Additionally, for operation in a data center, servers, particularly blade servers, need to be compact. Therefore, what is needed is a compact cooling solution adaptable for up to a large number of computers, that combines and balances air-cooling capacity for low-intensity heat sources with cooling liquid-cooling capacity for high-intensity heat sources while using a minimum amount of cooling liquid flow, and that is reliable, leak-free and low in power consumption.